1. Field of the Invention
The apparatus of the present invention relates to downhole oil well tools, and more particularly relates to an improved method and apparatus for setting a gravel pack in a downhole oil and gas well environment.
2. General Background of the Invention
There are a number of applications in the oil and gas well drilling industry where it is desirable to install a packer in an oil and gas well whose "annulus" or internal diameter is restricted by existing equipment. One downhole oil and gas well delivery system is known in the industry as a "coil tubing" unit. By using a coil tubing unit, it is possible to run a tool in a well that is very restricted in diameter because of existing equipment. However, there are many oil and gas well drilling operations that are not feasible heretofore with the small diameter coil tubing units.
Gravel packing is a mechanical means of preventing sand flow from unconsolidated formations in a producing well. If the sand flow is not controlled, serious and costly problems, such as, loss of production due to sand bridging, failure of casing or liners from removal of surrounding formation, compaction, erosion and disposal of produced materials. U.S. Patents that relate to gravel packs include U.S. Pat. Nos. 5,620,050 and 5,377,749 issued to Phil Barbee, applicant herein, each hereby incorporated herein by reference.
The purpose of a screen in gravel packs is to hold the gravel in place. The slot width or wire spacing should be smaller than the smallest gravel used. The outside diameter of the screen should provide maximum radial clearance of the casing wall while maintaining an adequate internal diameter for anticipated production rates. Screen sections should provide five feet of minimum overlap above and below the perforated interval to compensate for depth measurement inaccuracies. If the gravel is to be circulated into place, the screen may extend further above the perforated interval to develop a higher column of compacted gravel above the completion interval.
Two commonly used techniques for the placement of gravel are the "squeeze technique" and the "one trip circulating technique".
The squeeze technique is primarily used for gravel packing short intervals. Gravel is squeezed through the perforations to pack outside the casing and in the screen annulus without circulation. If the squeeze technique is used in longer intervals, variations of the formation permeability may cause all the slurry to go into the highest permeable section of the interval. Although longer intervals have been successfully squeezed, it is recommended that this technique be limited to shorter intervals.
A squeeze packer with a crossover tool is used to place the gravel pack. The screen and the blank pipe are run in the hole and positioned across the productive interval. The packer is set and the crossover opened. The slurry is then "bullheaded" down a workstring, through crossover tool, into the screen-casing annulus, and through the perforations in the casing. Pumping is continued until a pre-determined pressure increase or "sandout" pressure occurs, indicating that no more gravel can be "squeezed" outside of the casing or into the annulus. Once a "sandout" is achieved, pumping is discontinued and treatment pressures are vented before physically pulling the crossover tool to the "upper" circulating position. After the upper circulating position has been accomplished, pumping is resumed to circulate any excess gravel remaining in the workstring to surface.
The one trip circulating technique is typically better suited for longer intervals than the squeeze technique, but can be used for any length interval. A washpipe is positioned inside and extending through the screen to accommodate the circulation of fluids and gravel to the bottom of the screen. A gravel slurry is circulated down the tubing, through a crossover tool, down the screen-casing annulus, through the screen, up the washpipe, through the crossover tool and returns up the workstring-casing annulus. Gravel contained in the slurry is separated out of the circulating fluid as it passes through the screen.
As the screen is covered with gravel, the circulation pressure increases, forcing gravel into the perforations. Pumping is continued until a pre-determined pressure increase or "sandout" occurs indicating that no more gravel can be circulated outside of the casing or into the annulus. Once a "sandout" is achieved, pumping is discontinued and treatment pressures are vented before pulling the crossover tool to the "upper" circulating position. After the upper circulating position is accomplished, pumping is resumed to circulate out any excess gravel remaining in the workstring.
Slurry rates may vary as low as 0.25 bbl/min to in excess of 16.0 bbl/min depending on workstring or tubular diameters and the well configuration. Higher rates result in higher treating pressures which generally promote tighter packing of gravel. At higher placement rates, friction pressure is incurred due to pumping the slurry down relatively long lengths of tubing. These friction pressures tend to "mask" the actual down-hole differential pressure from the screen exterior to the screen interior during a "sandout".